1. Field of the Invention
The present invention relates to a cap with an induction seal closure for sealing the pouring opening of a container, and to an injection-moulded screw cap in particular.
2. Description of the Prior Art
Injection-moulded screw caps are commonly used for sealing bottles or jerricans. The main functions that screw caps are requried to perform are keeping the bottle or jerrican closed until the contents must be used, and offering the possibility to open the bottle without difficulty and to re-close it properly with ease when only part of the contents is used at a time.
A particular form of screw closure is known which is devised to operate in conjunction with containers that are initially sealed by an induction-sealed membrane. Such membranes typically consist of a foil of aluminium bonded to a thermoplastic layer such as a layer of LDPE (low-density polyethylene), the thermoplastic layer being sealed to the opening of a container by its melting caused by inductive heating of the aluminium foil. This seal insures the user that he is the first user of the container and it also forms a safe leak-tight closure of the container during all the phases that precede its first use, such as manipulations in a warehouse, transport by road or by air, etc. Finally, this seal allows the container to be positioned in apparatus in which the container is used upside down, and emptying of the container starts by perforation of the membrane as the container becomes located on its seat.
A disadvantage of an induction seal closure is that removal of the closure cap from the mouth of the container can leave fragments of the aluminium-LDPE laminate adhered to the rim of the container mouth. This hampers easy recycling of the container since fragments of aluminium in the recycled plastic precludes a homogeneous extrusion blow-moulding of products from the recycled plastic.
It has been proposed to provide the induction seal closure with a releasing layer which causes a reduction of the bonding strength between the aluminium foil and the thermoplastic sealing layer after the sealing has been carried out, so that the initial unscrewing of the cap from the container causes the aluminium foil to become detached from the container and to remain in the screw cap. The problems with recycling the empty container are solved in this way but now the aluminium remains within the cap and so forms a problem for recycling the screw caps.